Field of the Invention
The present invention relates to a magnetic carrier to be used in an image-forming method including the step of developing (visualizing) an electrostatic latent image (electrostatic image) through use of an electrophotographic method.
Description of the Related Art
Higher speed and higher reliability of a copying apparatus or a printer have been strictly sought in recent years. Meanwhile, the copying apparatus and the printer have started to be constructed of simpler components in various respects. As a result, performance demanded for a developer has become more sophisticated. Accordingly, unless art improvement in the performance of the developer can be achieved, a more excellent copying apparatus and a more excellent printer do not become viable nowadays.
Of the methods each involving developing an electrostatic latent image formed on an electrostatic latent image-bearing member with a toner, a two-component developing method involving using a two-component developer obtained by mixing the toner with a magnetic carrier has been suitably employed in a full-color copying machine or printer required to provide high image quality. In the two-component developing method, the magnetic carrier provides the toner with a proper quantity of positive or negative charge through triboelectric charging, and the magnetic carrier carries the toner on its surface by means of the electrostatic attraction of the triboelectric charging.
Although various characteristics are demanded for the magnetic carrier and the toner constituting the two-component developer, characteristics particularly important for the magnetic carrier are, for example, proper charge-providing ability, resistance against an alternating voltage, impact resistance, wear resistance, resistance against spent toner, and developing ability.
The magnetic carrier has characteristics, such as powder characteristics, electrical characteristics, and magnetic characteristics, and is required to have each performance to fit with a developing system. In recent years, a magnetic carrier obtained by coating a core material (core) with a coating resin (coating material) has been widely used for improvements in environmental stability and durability.
For example, a magnetic carrier having at least two coating resin layers is used in each of the two-component developers of the following patent literatures.
In Japanese Patent Application Laid-Open No. 2005-49478, there is a disclosure of the following magnetic carrier. The resin of the outermost surface layer of the carrier contains a condensate of an N-alkoxyalkylated polyamide and a silicone resin, and the carrier has an intermediate layer containing a resin containing fine particles between the resin of the outermost surface layer and a core. Thus, charging stability under long-term use and the wear resistance of a coating film are improved, and hence the durability of the two-component developer is improved.
In addition, in Japanese Patent Application Laid-Open No. 2004-333931, there is a disclosure of a magnetic carrier that provides a toner image excellent in fine-line reproducibility through an improvement in film quality of a resin layer by the incorporation of ferrite particles subjected to a lipophilic treatment into the undermost layer of a coating resin.
In addition, in Japanese Patent Application Laid-Open No. 2008-70662, there is a disclosure of a carrier that expresses the following effect through the incorporation of alumina fine particles subjected to a hydrophobic treatment into a first coating resin layer coating the outer periphery of a core material. Spent components on the surface of the carriers are mutually shaved off by the carriers. As a result, a coating resin that has not received any stress is always exposed to the surface, and hence the performance of the carrier is retained and excellent life stability is achieved.
In addition, in Japanese Patent Application Laid-Open No. 2007-121911, there is a disclosure of a carrier including an inner resin coating layer with which the surface of a core particle is coated and an oater resin coating layer with which the surface of the inner resin coating layer is coated, the inner resin coating layer containing nonmagnetic fine particles whose surfaces are coated with fatty acid metal fine particles. As a result, even in the case of development over a long time period, an increase in van der Waals force between the core particle and a toner can be prevented, and hence charging performance can be stably maintained.
In addition, in Japanese Patent Application Laid-Open No. 2009-229907, a carrier for electrophotography in which carbon black is present at an interface between a first coating resin and a second coating resin sequentially formed on a magnetic material particle is used to solve a problem in that a coating resin peeled by wear migrates to a fixed image to make the tint of the fixed image turbid.
Nowadays, however, a burden to be imposed on a developer in a developing device tends to increase owing to, for example, a reduction in developer volume in association with the downsizing of the developing device and an increase in stirring speed of the developer by an increase in output speed thereof. As a result, particularly under a high-temperature and high-humidity environment, the spending of a toner or an external additive onto the surface of a magnetic carrier based on a water crosslinking force acting between the magnetic carrier and the toner progresses, and hence the charge-providing ability of the magnetic carrier reduces. In addition, the adsorption of moisture to the surface of the magnetic carrier progresses to temporarily reduce the strength of the coating resin of the magnetic carrier, and hence the shaving of the coating resin of the magnetic carrier occurs to reduce its charge-providing ability.
Even when the developer disclosed in Japanese Patent Application Laid-Open No. 2005-49478 or Japanese Patent Application Laid-Open No. 2004-333931 is used, a crack occurs in a coating resin on the surface of the magnetic carrier owing to a severe burden on the developer in a recent developing device, and hence a wax derived from a toner adheres to the crack portion in some cases. As a result, a fine particle derived from the toner adheres to the portion of the surface of the magnetic carrier to which the wax adheres, and hence the magnetic carrier cannot maintain carrier characteristics in its initial state and has insufficient durability. In addition, moisture adsorption progresses from the crack portion occurring in the coating resin on the surface of the magnetic carrier to reduce its charge-providing ability. As described above, even the carrier disclosed in Japanese Patent Application Laid-Open No. 2005-49478 or Japanese Patent Application Laid-Open No. 2004-333931 is susceptible to further improvement.
In addition, in the case of each of Japanese Patent Application Laid-Open No. 2008-70662, Japanese Patent Application Laid-Open No. 2007-121911, and Japanese Patent Application Laid-Open No. 2009-229907, the shaving of a coating resin cannot be completely prevented.